The present invention relates to a method for measuring the amount of substance associated with a material in the presence of a contaminant, and more particularly to a method for measuring the amount of water associated with a paper material in the presence of carbon.
Methods for detecting moisture in paper material are well known in the prior art, see e.g., U.S. Pat. No. 3,614,450. Typically, an infrared source emits two bands of electromagnetic radiation. A first band (usually 1.8 microns--so called reference channel) is insensitive to absorption by the moisture. A second band (usually 1.94 microns--so called measure channel) is sensitive to absorption by the moisture. The two bands of radiation are directed at the paper material. Detectors are positioned to receive the bands of radiation after they have been reflected from the paper or transmitted through the paper. The detectors convert the radiation received into electrical signals. The ratio of the signal of the reference channel to the measure channel is indicative of the moisture content of the paper material. A fundamental assumption of this method is that neither the reference channel nor the measure channel is sensitive to absorption by other compounds in the paper material.
As ecological demands increase, the use of recycled paper also increases. This has occurred most frequently in the news print industry. Used newspaper is recycled with new pulp to produce fresh news print. The use of recycled news print in the manufacturing process introduces contaminant, namely carbon from the printing ink, into the process. The presence of carbon affects the measurement of moisture of the paper material in that the bands (both reference channel and measure channel) of electromagnetic radiation are absorbed by the carbon. Thus, the ratio of the signals of the reference channel to the measure channel would not be determinitive of the moisture content of the paper material.
Heretofore, one way to correct for the presence of carbon is to determine a priori the influence of carbon on the ratio of the signals for a particular moisture level. For example, at 7% level of moisture for a particular paper material, a ratio of the signals without carbon was determined to be 2.2 and as carbon was introduced the ratio of the signals detected, for the same level of moisture, became 2.0.
For a different level of moisture, the ratio of the signals would also vary as the amount of carbon present in the paper material. In this manner, a family of curves was pre-determined and usually stored in a computer. The amount of carbon present in the paper material was determined from the change in the signal strength of the reference channel. Since the reference channel was insensitive to the presence of water, the change in signal strength of the reference channel could be attributed to the presence of carbon. With knowledge of the ratio of the signals and the amount of deviation of the signal strength in the reference channel, indicating the amount of carbon present, the moisture level of the paper material could thus be calculated from the family of pre-determined curves. The drawback of this method, of course, is that a large amount of data had to be pre-determined and stored in a medium which is easily and quickly accessible. Moreover, the method that was developed was based upon empirical results and not upon theoretical basis. As a result, it was limited in its accuracy and in its range of application.